Stot-type high-intensity X-ray source

ABSTRACT

An x-ray radiation source in which a focused electron beam impinges upon a target positioned in front of the focal point of the electron beam and the radiation emitted by the target travels through the aperture of an aperture diaphragm with the aperture disposed at the focal point of the electron beam.

The proposed invention belongs to the class of X-ray sources allowing toobtain intensive X-ray radiation with small effective dimensions of theradiation emission region. The invention is intended to be used in X-raymicroscopy, micro-defectoscopy, computer tomography, etc.

The X-ray radiation is known to be generated when an electron beamemitted by a cathode and accelerated by the electrode potential bombardsan anode. When electrons decelerate in some material, X-ray radiation isgenerated. As investigation of X-ray radiation beam pattern emitted byan X-ray tube anode shows, the primary direction of the softer radiationcomponent is perpendicular to the electron beam direction, while thedirection of the harder component is close to that of the electronsfalling onto the anode. As the applied voltage increases, the diagram ofthe X-ray radiation spatial distribution becomes more narrow-angled.

To either achieve high definition or to enlarge an image of the X-rayedobject region (X-ray microscopy), radiation sources with small effectivedimensions of the radiation emission regions, usually micro-focal X-raytubes, are used. The target region out of which X-ray radiation isemitted when the target is bombarded by the electron beam is called thefocus. The X-ray tubes in which the focus dimensions do not exceedseveral microns (or tens of microns) are called micro-focal. The focalspot dimensions are determined by the electron beam focusing extent, thetarget material, and the X-ray source design. The focal spot dimensionsand the source radiation intensity that can be achieved are primarilydependent on the target material thermal resistance. Since a largeamount of heat is released within a finite space when the electron beamdecelerates in the target material, the target may be destroyed; this isthe so-called thermal limit of the focal spot dimensions for the givenspecific load. On the other hand, the focal spot dimensions can not bemade infinitely small due to electron scattering within the targetmaterial because this scattering increases the dimensions of the X-rayradiation emission region; this is the so-called electron limit.Increasing the tube radiation intensity, simultaneously making the focalspot dimensions smaller, is almost always a difficult task because thesmall focal spot dimensions do not allow to increase the electron beamintensity due to the target material destruction caused by release ofgreat amount of heat. In certain X-ray tubes with 1-micron focal spots,the released power is about several hundredths of a Watt; the power is0.6 Wt for 5-micron dimensions of the focal point. One more problem ofmanufacturing micro-focal X-ray tubes is to achieve the short focaldistance, i.e. the distance between the X-ray tube focus and the X-rayradiation output window. To implement this, the tubes with transparentanodes are used. In these tubes, X-ray radiation is emitted from thetarget side opposite to the side of the electron beam incidence.

To make the anode focal spot small, the focusing accessories such aselectrostatic, magnetic, and electromagnetic lenses are used; todecrease the thermal load on the anode focal spot with small dimensions,anode scanning by the electron beam is used as well as devices for anoderotation.

A micro-focal X-ray tube is known in prior art in which electronsemitted by the cathode are focused by the electron lenses into a pointon the anode. The three-layer anode contains the target made of foil togenerate X-ray radiation, the layer for electron deceleration, and thesupporting substrate due to which the anode also serves as theX-ray-tube window. In this tube, the anode is transparent. To avoidanode damage at the point of electron beam incidence, the anode isconnected to the engine providing anode rotation, so electrons fall ontodifferent anode regions. (See Application PCT No. WO 96/29723, H01J35/08, 35/24, publ. in 1996).

A high-power X-ray tube is described in the German patent No. 2441986,H01J 35/04, publ. in 1975. The tube is an evacuated chamber with theradiation output window, inside which an incandescent cathode ispositioned as well as a transparent anode in the form of a cone, withthe cone vertex directed towards the cathode. Electronic-opticalaccessories control the electron beam, thus providing uniform anodeload.

In the German application No. 3543591 A1, H01J 35/22, publ. in 1986, apulse micro-focal X-ray tube is described comprising the cathode, theelectron lens to focus the electron beam, and the anode eithertransparent or massive and cooled, with a target to generate X-rayradiation. In this case, X-ray radiation comes out through a berylliumwindow at 90° angle with respect to the direction of the electronincidence.

The X-ray radiation source is also known which is the evacuated chamberwith the window for X-ray radiation output, inside which the cathode andthe anode are positioned. The source also contains the device directingthe narrow electron beam to fall onto the anode and the deflectingdevice scanning the anode. The anode is transparent and has thefollowing design: the target is a thin layer of a metal, for example,copper, deposited by vacuum spraying onto a thin substrate made of ametal with a relatively small atomic number, for example, aluminum. Theplate made of a small-atomic-number material, for example, plastic,serves as a holder for the substrate; a multi-aperture cellularsupporting structure is also present in the construction. Such designprovides high transmission of X-ray radiation generated by the target.The chamber is placed into a collimating device allowing to properlyshape the X-ray radiation beam (a source of this type is described inthe U.S. Pat. No. 4,057,745, Cl. H01J 35/08, publ. in 1977). Thattechnical solution is closest to the one proposed herein.

The invention purpose is to create the X-ray radiation source providingdecrease of the radiation-emission region effective dimensions forsufficiently high radiation intensity and short focal distance.

The conventional methods in which the anode is scanned by the electronbeam or rotated are not used to decrease the anode load. The electronbeam is proposed to be focused behind the anode, and an X-ray-beamdiaphragm is proposed to be positioned at the focus of the electronlens. As the defocused electron beam falls onto the anode, the anoderadiation load decreases, thus allowing to increase the acceptableelectric power. Due to X-ray radiation beam pattern formed for suchgeometry and to positioning the diaphragm at the electron lens focus,the obtained radiation is similar in its characteristics to that of amicro-focal source positioned at the location of the diaphragm andhaving the corresponding focal-spot dimensions.

The invention essence is that, in the well-known technical solution,which is an X-ray radiation source comprising an evacuated chamber witha window for X-ray radiation output and with an electron emitter and atransparent anode positioned in the window to generate X-ray radiation,at least one focusing electron lens, and a device shaping the X-rayradiation beam placed outside the chamber but attached to it, the anodeis positioned before the electron lens focus along the electron beampath while the device shaping the X-ray radiation beam is a diaphragm,the center of which is placed at the focus of the electron lens.

To reduce the X-ray radiation losses, the anode can also serve as theX-ray-tube window. In this case, to increase the structural strength,the anode is implemented as a target made of metal foil deposited onto asubstrate made of a small-atomic-number material with high heatconductivity. The anode may also be tightly vacuum-attached to thewindow for X-ray radiation output and positioned inside that window. Theelectron lens may have either a point or a dash-like focus, depending onproblems to be solved. When the anode is the X-ray-tube window, theanode can be equipped with a cooling facility. The electron source usedmay be a pulse source.

The invention essence is explained by the following drawings:

FIG. 1, the beam pattern of radiation of the X-ray tube with thetransparent anode is shown for different anode-cathode voltage values(U₃>U₂>U₁).

In FIG. 2, the direction of the electron beam incidence and theX-ray-radiation beam pattern for the proposed source are shown.

In FIG. 3, the layout overview for the proposed X-ray-radiation sourceis schematically presented.

FIG. 2 illustrates that the spatial distribution of radiation emitted bythe proposed source is identical to that of radiation of a micro-focalsource positioned at the location of the diaphragm, with the anode rayload reduced (the beam is defocused on the target). This figure showsthe electron beam 1 that falls onto the target 2 that generates X-rayradiation 3 converging towards the diaphragm 4, the aperture of which isplaced at the electron lens focus (not shown in this drawing). Item 6indicates spatial distribution of X-ray radiation at the output of theproposed source.

Consider operation of the device shown in FIG. 3. Electrons emitted bythe cathode 7 (e.g. a thermocathode; however, this is not essential) areshaped by the focusing cap into a beam and are focused by the electronlenses 9 and 10 on the anode 11, which is the target 12 made of metalfoil deposited onto the substrate 13 made of small-atomic-numbermaterial (the target can be deposited onto the substrate by vacuumspraying). The substrate provides the structural strength and heatremoval and can be conveniently fixed to the source chamber so as theanode can also serve as the window for X-ray radiation output. However,the anode made of foil can be used without the substrate. In this case,the chamber is supplied with a beryllium window for X-ray radiationoutput (not presented in the drawing). The anode and the cathode arepositioned within the evacuated chamber 14. The diaphragm 15 shaping theX-ray radiation beam is positioned outside the chamber and behind theanode. The diaphragm can be attached to the source chamber 14. Thecenter 16 of the diaphragm 15 must be placed at the focus of theelectron lens 10. The electron lens 10 can have either a point or adash-like focus, depending on the problems to be solved using theequipment in which the proposed X-ray radiation source is implement.When the anode is the source output window, it may be equipped with thecooling facility 17.

What is claimed is:
 1. An x-ray radiation source comprising an evacuatedchamber with a window for x-ray radiation output, in which an electronemitter and a transparent anode are positioned to generate x-rayradiation, at least one focusing electron lens, and a device shaping thex-ray radiation beam placed outside the chamber but attached to it,wherein the anode is positioned before a focus of said electron lensalong the electron beam path and wherein the device shaping the x-rayradiation beam is an aperatured diaphragm, the center of said aperaturebeing placed at the focus of said electron lens.
 2. An x-ray radiationsource of claim 1, wherein the anode is a target made of metal foildeposited onto a substrate made of a small-atomic-number material.
 3. Anx-ray radiation source of claim 1 wherein the anode is tightlyvacuum-attached to the window for x-ray radiation output and positionedinside that window.
 4. An x-ray radiation source of claim 3, wherein theanode is equipped with a cooling facility.
 5. An x-ray radiation sourceof claim 1, wherein the electron lens has a point focus.
 6. An x-rayradiation source of claim 1, wherein the electron lens has a dash-likefocus.
 7. An x-ray radiation source of claim 1, wherein the electronsource used is a pulse source.